1. Field of the Invention
The present invention relates to a digital signal demodulator that directly converts radio frequency signal modulated by phase shift keying to a digital signal and a wireless receiver using the same.
2. Description of the Related Art
In recent years, a mobile communication appliance such as portable cellar telephone is sophisticated in a service in accordance with rapid market expansion. The spread of wireless LAN systems realizing a high-speed data transfer by wireless is also remarkable. Generally, in such wireless communication systems, the digital modulation scheme using the high carrier wave of a GHz band is used.
The digital modulation scheme can be classified into amplitude shift keying (ASK), a phase shift keying (PSK), and a frequency shift keying (FSK). Among them, the phase modulation scheme represented by PSK that uses phase of a carrier wave for digital signal modulation is the most commonly used. Receiver circuits for receiving and demodulating a high frequency signal can be classified roughly in a heterodyne scheme and a direct conversion scheme.
In a conventional technique, a band selection filter is used for filtering a specific frequency band referred to as a system band in the receiver of either scheme. The system band is a frequency band assigned to a user based on a certain communication standard. The system band contains a plurality of channel bands of a narrower band width, each of which is assigned to each user.
In a conventional receiver, at first, the received high frequency signal is passed through the band selection filter to select the system band, and then converted into a signal of an intermediate frequency band or baseband with a down-converter. The received signal subjected to the frequency conversion is further converted into a digital signal with an AD converter. The digital signal is subjected to a digital signal process or passed through a digital filter to extract only a digital signal included in the channel band assigned to every user.
The above conventional receiver requires analog circuits for processing a high frequency signal such as a down converter as well as a band selection filter and a channel selection filter, resulting in a complicated and large-scale circuit. Such a complicated circuit configuration is required because it is impossible to realize a simple demodulator configuration which can extract directly a demodulated output digital signal from high frequency signal with a GHz band carrier without using a filter or a down-converter mixer circuit.
On the other hand, there has been developed a receiving IC capable of directly sampling and demodulating a received signal of 2.4 GHz using a CMOS device which is used for a digital signal process or for a logic circuit as described in IEEE International Solid State Circuit Conference (ISSCC2004) Digest of Technical Papers, pp. 268-269.
The above receiving IC can directly demodulate a high frequency signal of 2.4 GHz into a digital signal. However, since the receiving IC does not provide function for extracting a specific frequency component from the received signal, it is still requires a channel selection filter.
It is an object of the present invention to provide a digital demodulator capable of demodulating a radio frequency signal modulated by phase shift keying into digital signal with a simple circuit configuration, and a receiver providing with the same.